This invention relates generally to semiconductor processing and, more particularly, to a system and method for monitoring a wafer""s location during processing.
In general, integrated circuit technology is based on the ability to form numerous transistor structures on a single semiconductor substrate. Typically, multiple integrated circuits will be formed on a single silicon wafer, the wafer providing the semiconductor substrate for the circuits. To increase the purity and hence the quality and reliability of the substrate, the wafer may be pre-processed to form an ultra-pure epitaxially grown (xe2x80x9cepixe2x80x9d) layer on top of the wafer. Such preprocessing is typically performed by the following steps:
1. an unprocessed wafer is loaded onto a loadlock;
2. the wafer is then transferred from the loadlock to a center-find device;
3. the wafer is then transferred from theter-find device to a process chamber;
4. the wafer is then transferred back to the load lock.
In each case, a robot arm is used to move the wafer from one chamber to another, as well as to position the wafer inside the process chamber. As a result, substantial savings in cleanliness, time, and labor expense are achieved. However, inaccuracies sometimes occur when using a robot arm which can degrade the wafer and cause substantial yield loss. For example, the wafer is moved on the robot arm under an optical assembly, or center finder. The center finder finds a geometric center of the wafer. The location of the geometric center of the wafer can then be used by the robot arm to assure that the geometric center of the wafer is placed in the desired location in the process chamber.
This is accomplished by the following method. First, the location, which is represented as an offset between the actual center of the wafer and the nominal center, that is where the center of the wafer should be. Next, the offset is scaled to provide an offset value to the robot arm so that it can place the center of the wafer in a desired location in the process chamber.
However, several problems often resulted from the above describe method. For one, the wafer may move on the robot arm or while it is being loaded into the process chamber, thereby making the offset value incorrect. For another, the wafer may move inside the process chamber while it is being processed. Finally, the wafer may move during or after it has been unloaded from the process chamber. Each of these problems are especially troublesome because the only way of detecting the problem is by a resulting process degradation, yield loss, or in severe situations, machine failures. In the case of process degradation and yield loss, these are often detected only after many wafers have been incorrectly processed.
The foregoing problems are solved and a technical advance is achieved by a system and method for monitoring a process flow of a semiconductor wafer. In one embodiment, the method calculates a first offset value of the wafer before it is processed. The wafer is then moved into a process chamber where it is processed. Then a second offset value of the wafer is calculated before the wafer is unloaded. If the difference between the first and second offset values are within a predetermined amount, the wafer is unloaded and regular processing continues. If the difference is not within the predetermined amount, an alarm is activated and the process is stopped.
An advantage of the present invention is that it provides an early indication of process degradation or yield loss. Such early indication would result in higher product quality and higher process yields.
Another advantage of the present invention is that it can prevent any loss or damage that may occur when the wafer is being unloaded.